1. Field of the Invention
The present invention relates to a method for determining the number of healthy strings in each block for each module of a sodium-sulfur battery and a method for detecting a failure for each module of the sodium-sulfur battery by using the same.
2. Description of Related Art
A sodium-sulfur battery is expected to see a wider use as an electric power storage device in application fields, such as electric power load leveling, measures against a momentary drop in power, and compensation for fluctuations in the power generated by a renewable energy generating device.
In the application of the load leveling, the operating cycle of the sodium-sulfur battery consists of, for example, the repetitive cycle of a discharge during the daytime and a charge during the nighttime with a rest between the cycles. In this case, the sodium-sulfur battery reaches the vicinity of the end of a discharge once a day after the discharge during the daytime. Meanwhile, in the application of compensation for fluctuations in electric power, the sodium-sulfur battery repeats a charge and a discharge within a short time so as to compensate for an output (electric power) of a renewable energy generating device.
The aforesaid sodium-sulfur battery is generally configured by connecting a plurality of electric cells in series to form a string, and then connecting a plurality of the strings in parallel to form a block. Further, a plurality of the blocks is connected in series to form a module, and then a plurality of the modules is connected in series to complete the sodium-sulfur battery.
Failure of the sodium-sulfur battery to exhibit its expected performance leads to a problem in the aforesaid applications, such as the load leveling measures against a momentary drop in power, and the compensation for fluctuations in power. It is, therefore, extremely important to detect a failure of the sodium-sulfur battery.
Hitherto, a failure of the sodium-sulfur battery is detected by measuring the voltage of each block when the voltage of the battery is stabilized during the period of a rest from the completion of a discharge to the start of a charge and then by determining whether the voltage of the block is out of a set range or whether a voltage difference between blocks is out of a set range. As described above, the block is formed by connecting in parallel the strings, each of which is formed by connecting electric cells in series. As the documents on prior arts, patent documents 1 and 2, for example, may be cited.